The invention relates to a method in connection with reverse stranding, wherein conductors for a cable to be produced, such as wires, groups or blocks, are drawn from supply reels or the like through a divider means, torsion tubes peripherally surrounding a central element and periodically rotatable in opposite directions about the central element, and a twisting head rotatable in opposite directions, into a nozzle or the like.
In traditional reverse stranding, i.e. SZ stranding, conductors are drawn by a suitable drawing apparatus through a stationary divider means and a twisting means rotating periodically in opposite directions into a nozzle, thereafter the conductor is immediately bonded in a bonding device into a reversely stranded product, such as a cable. After the drawing apparatus, the cable is reeled, or the stranded cable is passed to the next production stage. In a traditional reverse stranding apparatus of this type, it is usual to position a tubular intermediate element rotating with the twisting means between the stationary divider means and the rotating twisting means. The intermediate element is attached centrally to the twisting means and mounted at one end rotatably with respect to the stationary divider means.
This traditional arrangement, however, has the disadvantage that the rotation rate of the intermediate element between the divider means and the twisting means is constant, and therefore the twist in the conductors tends to accumulate at the end close to the twisting means. The pitch angle of the conductors thereby gets greater, and the angle deviation of the conductors in the twisting means increases. As a consequence, a greater force is required to draw the conductors, and so the stranded conductors tend to untwist.
Several different solutions have been suggested to the above problem. FI Patent Specification 78576, corresponding to U.S. Pat. No. 4,974,408, for instance, discloses one prior art solution.
The solution disclosed in FI Patent Specification 78576 works well in practice although it has certain disadvantages. A problem with the SZ twisting is that the friction increases with the twisting angle of the oscillating divider plate, and so the tension caused in the wires, conductors, groups or blocks varies. As a result, the pitch length varies, and locking at the direction reversion point becomes more difficult to carry out. This is significant particularly with telecommunication cables as the interference tolerance of the group increases with the degree of symmetry of the pair or quad. Such interferences include cross-talk, external interferences, etc. Wires are subjected to jerks already when they are unwound from the supply reels. Each bending over the idler wheels increases the tension of the wires. The tension of the wires varies within a wide range especially when unreeling "over the flange". Before the grouping or stranding means, the tension of each wire is different, and it cannot be levelled out by the brakes provided at the inlet end. The friction increases with the twisting angle in the tube packet of the SZ torsion tube stranding means, and at the same time the tube packet gets shorter. In addition to the variation in tension, there occurs variation in the speed of the wires. For locking the stranding, a pitch shorter than the nominal pitch is used on both sides of the reversion point. This is called edge acceleration. The shortness of the locking pitch and the number of turns used, i.e. the effective length, determine the magnitude of the speed variations acting on the wires between the twisting head and the supply reels. This factor is particularly apparent after the stranding point. At the direction reversion point, i.e. at the edge acceleration stage, an extra wire length is instantaneously needed at the stranding point. Extra length is obtained from the supply side as well as from the side of the finished group. The attempt to obtain extra wire length after the stranding point results in the occurrence of slipping on the capstan and in a pumping effect acting on the finished group between the grouping means and the standing means. This can be seen from the fact that the lengths of the grouping or stranding pitches vary continuously, thus deteriorating the properties of the cable.
Sheathing lines usually employ a cable stranded helically at a separate production stage. In the sheathing process, tension is exerted on the cable by means of two belt drawing devices, one of which is positioned before the press and acts as a braking and/or drawing means while the other acts as a primary drawing means and is positioned at the terminal end of the line. The cable tension used in this kind of system is too high for SZ stranded cables. Excessive tension opens the direction reversion points in the SZ stranded cable and, in the worst case, straightens all individual conductors. In the torsion tube SZ stranding, the friction exerted on the conductors varies with the twisting angle of the tube packet, being at the greatest at the direction reversion point. This causes tension peaks tending to untwist the SZ stranded cable before the sheathing press. In various SZ stranding point locking systems, such tension variations cause stranding errors, or these systems can be applied only within a very limited operating range. By special arrangements, such as by stranding between the belts of the brake drawing device, passable results can be obtained. The tension of the cable is adjusted by guiding the brake drawing device by a suitable device, such as a so-called dancer. Many solutions of this type are known in the art, but a feature common to all of them is that they are complicated and provide unsatisfactory results.
The object of the invention is to provide a method and an arrangement by means of which the disadvantages of the prior art technique can be eliminated. This is achieved by means of a method according to the invention which is characterized in that the conductors are guided so as to pass about an input capstan before the divider means; that a stranded group, strand or cable is passed about an output capstan immediately after the nozzle; and the input capstan and the output capstan are rotated so that the peripheral speeds thereof are constantly higher than the speed of the wires, groups, strand or cable. The arrangement according to the invention, in turn, is characterized in that it comprises an input capstan which is positioned before the divider means and about which the conductors are arranged to pass; and an output capstan which is positioned immediately after the nozzle or the like and about which the stranded group, strand or cable is arranged to pass; and that the input capstan and the output capstan are arranged to be rotated so that The peripheral speeds thereof are constantly higher than the speed of the wires, groups, strand or cable.
An advantage of the invention is that it enables the pitch lengths to be maintained at the preset values during the SZ stranding or grouping. The tension exerted on the cable, groups or conductors after the apparatus is negligible. The shape of the direction reversion point can be adjusted by edge acceleration. Variation in the pitch length during the grouping is less than 2% when the rotation rate of the twisting head and the line speed are constant. In an apparatus for stranding telecommunication cables, two SZ torsion tube stranding machines can be arranged in succession so that the first produces the pairs or quads while the second strands them together. After each production stage, substantially all of the tension acting on the wires, conductors or groups is removed. Due to the high symmetry of the group, the electrical values of finished telecommunication cables will be excellent. The telecommunication cables will also be of high quality as the method does not cause the wires or conductors to be stretched nor does it damage the insulation at any stage. A further advantage of the invention is that it is advantageous in price as only the primary drawing device is required in the sheathing line in place of the expensive brake belt drawing device and associated guiding means. The stranding and sheathing speeds can be increased to hundreds of meters per minute without any detrimental effects on the stranding process. No straight parts are required at the direction reversion points of the stranding process, but these parts are curved. The combined effect of the nozzle and the capstan eliminates any tension peaks created in the cable stranding. The magnitude of the tension can be controlled by adjusting the slip between the cable and the capstan. The constant braking force exerted on the cable may also be increased and decreased in an advantageous manner. Tension variations can also be levelled out after the capstan even though the friction exerted on the conductors increases with the twisting angle of the tube packet, being at the greatest at the direction reversion point.